Apparatus and method for computing drilling costs

ABSTRACT

Apparatus and method for computing the cost per unit length of borehole drilled by a drilling apparatus, as for example, the cost per foot of borehole drilled. First signals representative of incremental lengths of an interval of borehole depth drilled and a second signal representative of the accumulated cost incurred in drilling the interval of depth are generated and used to generate a third signal representative of average cost of operating the drilling apparatus during the interval of depth. The third signal may be read out and may be graphically recorded as a function of the depth drilled during the interval.

O-O2-72 XR 396609649 Elite States atent 1151 3,660,649 Gilchrist et al,51 May 2, 1972 [s41 APPARATUS AND METHGD FOR 2,669,87l 2/1954 Lubinski..23s/1s4 x COMPUTING DRILLING O TS 3,324,717 6/1967 Brooks 6t 61....73/1s2 2,770,771 11/1956 Schuster ....235/193 x Inventors RalphGilchrist, Houston; 3,403,327 9/1968 Welz ....23s/193 x Brown, SanAntonio, both of TeX. 2,539,758 l/l95l Silverman et al. ..235/l 84 X[73] Assignee: Tenneco Oil Company, Houston, Tex. I

Primary Exammer-.loseph F. Ruggiero [22] F d p 1970 Attorney-Eugene S.Coddou [21] Appl.No.: 75,864

[57] ABSTRACT Apparatus and method for computing the cost per unitlength I 3 3 [52] U s C 2 5/193 7 5 5 3 of borehole drllled by adrilling apparatus, as for example, the [51] Int Cl G068 7 M8 B2) 45/00cost per foot of borehole drilled. First signals representative of 58Field 615661;iI1111111111111111111111535/193 151.3 184 61- im'emema'lengths imem" deph d'med 73/151 151 175/40, 346/33 250/83 and a secondsignal representative of the accumulated cost in- 340/1 166/64 6 324/1curred in drilling the interval of depth are generated and used togenerate a third signal representative of average cost of f operatingthe drilling apparatus during the interval of depth. [56] Re erences Thethird signal may be read out and may be graphically UMTED STATES PATENTSrecorded as a function of the depth drilled during the interval.3,364,494 1/1968 Dellinger et al ..73/ 1.5 X 14 Claims, 4 DrawingFigures 2 A e 72 I6 2/ 23 VIEW? a r R; ML L AD DER -IV DER AI 4P} 7' t 50 GE ADDER 7' c/Rcull h I L 6 77 l 7 TRIP TIME (It) i 26 CIRCUIT DOZLARS/ FOOT .RIG COST [6 (Cr) 19/7 cosr (o ciT/? C/RCU' T RECORDER 9 ,1 20DEPTH (a) DEPTH (d) PULSES STORAGE PATENTEDMAY 2 I972 3, 660,649

SHEET 1 BF 2 DRILLING COST DOLLARS /FOOT FEET OF DEPTH OUTPUT TIMEVOLTAGE -D/G/TAL REGISTER ER MANUAL RESET R7 R2 E 7 OPERATIONAL AMP.

Morton E. Brown FIG. 4

Ralph E. Gilchrist INVENTORS BACKGROUND OF THE INVENTION 1. Field of theInvention This invention relates to apparatus and method for determiningthe cost per unit length of borehole drilled by a drilling apparatus.

2. Description of the Prior Art The cost of drilling a borehole into theearth with a drilling apparatus in a particular location is a functionof the cost of the drill bit, the cost of operating the drillingapparatus per unit length of time, and the speed with which the drillbit penetrates the subterranean formations. Since the drill bit is aconsumable apparatus which decreases in efficiency with use and must bereplaced periodically, the total drilling time for producing a boreholeincludes actual bit penetrating time and the trip time required toremove the drill string from the borehole, replace the bit and resumedrilling.

Heretofore, a drilling operator was required to exercise his ownsubjective judgment based on his past experience in determining when adrill bit was consumed to the extent sufficient to justify the expensiveand non-productive cost of 1 replacing the drill bit. If the drillstring is tripped out of the borehole either too soon or too late, theover-all average cost per foot of drilling is increased. In order tominimize drilling costs, it is therefore important to be able toaccurately determine the point at which drilling efficiency decreaseswith continued drilling. No prior art apparatus or method is availableto accurately indicate instantaneous drilling efiiciency or to determinethe point of maximum drilling efi'rciency.

SUMMARY OF THE INVENTION The apparatus of this invention for determiningthe cost of drilling a borehole per unit length of hole drilled, as forexamplc the cost in dollars per foot drilled, includes first means forgenerating a first signal representative of incremental lengths of aninterval of borehole depth drilled and second means for generating asecond signal representative of accumulated drilling costs incurred indrilling the interval of depth. Third means responsive to the first andsecond signals generate a third signal representative of the averagedrilling cost per incremental length of borehole drilled.

The second means preferably includes fourth means for generating afourth signal representative of total drilling apparatus time and fifthmeans for generating a fifth signal representative of drilling costduring the apparatus use time. The fourth means preferably includessixth means for generating a sixth signal representative of the rotatingtime of the drilling apparatus and seventh means for generating aseventh signal representative of trip time. Also included are eighthmeans for generating an eighth signal representative of the cost of thedrill bit utilized in drilling the interval of borehole depth.

The apparatus of this invention also preferably includes means forreading out the average cost per incremental length of borehole drilledand means for graphically recording the average drilling cost perincremental length of the interval of borehole drilled as a function ofthe depth drilled during the interval.

The method of this invention includes generating the abovedescribedfirst through eighth signals and graphically recording average drillingcosts per incremental length of drilling interval as a function of thedepth drilled during the interval.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings: 1

FIG. 1 is a typical graph illustrating the change in drillin cost as afunction of depth drilled during an interval of depth.

FIG. 2 is a block diagram illustrating the apparatus of the invention.

FIG. 3 is a block diagram showing in detail storage unit 4 of FIG. 2.

FIG. 4 is a block diagram showing in detail the circuit of trip timeunit 6, bit cost unit 14 and rig cost unit 10 shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, FIG.1 illustrates a typical graph of average drilling cost as a function ofdepth drilled over an interval of drilling. This curve is based upon therelationship:

D=C, (t,+t,)+ C ld where: 6

C,=rig cost in dollars per hour C, bit cost in dollars I, rotating timein hours t, trip time in hours t, t, total drilling time in hours ddepth of drilling interval in feet D= average drilling cost in dollarsper foot Rotating time (t,) is the actual drilling time on each bit.Trip time (t,) is the time expended during the changing of a bit; i.e.,the, time required to remove all drill pipe in the borehole, replace thedrill bit, and again run all drill pipe back into the borehole to placethe new bit on bottomhole. The sum of rotating and trip time (t, t,) istherefore the total rig time required for each drill bit.

Visual examination of the cost per foot curve of FIG. 1 illustrates theoptimum or minimum drilling cost for a particular drill bit. During theinterval between A and B, drilling costs are declining with accumulateddepth. The cost at point B appears to reach an optimum value at which nofurther decrease in cost is apparent. The interval between point B andpoint C reflects a slight increase in cost, indicating a decreasingdrilling efficiency. Point C can therefore represent the depth at whicha new bit should be installed in order to operate at near optimumdrilling efficiency.

The apparatus and method of this invention allow automatic computationof average drilling cost per incremental length (e.g., per foot) ofborehole drilled from the relationship:

and provide an output which can be read out or indicated on a meter andcan be applied to a strip chart recorder to produce a curve similar tothat shown in FIG. 1 for each drill bit used. After a new bit has beeninstalled, but before the drilling is resumed, rig cost (C,), trip time(n) and bit cost (C,,) are entered into the apparatus as knownquantities. Rotating time (t,) is a real time variable, but is suppliedby an internal source. Borehole depth (d), the second real timevariable, must be supplied by an external source.

The cost per foot computer is therefore an electronic analog devicerequiring an input depth signal for each foot of borehole drilled andproviding an output voltage proportional to the instantaneous value ofthe curve of FIG. 1. The preferred embodiment of the cost per footcomputer apparatus of this invention is shown in block diagram in FIG.2. Depth pulses 18 represent the only external variable input required.

Depth pulses 18 may be supplied by any convenient signal sourcecompatible with this apparatus and with strip chart recorder 33. Onesuitable source for pulses 18 is manufactured by W. & L. E. GurleyCompany of Troy, N. Y., and bears model No. 8602-1. Pulses 18 are storedin depth storage unit 19, as described below, and are supplied to stripchart recorder 33 which is used to record the output from the computerapparatus and to produce a curve similar to FIG. 1. Recorder 33 may beof any compatible type which is pulse driven in the horizontal plottingdirection in order to provide horizontal chart movement proportional toborehole depth increases. One convenient type of recorder 33 ismanufactured by Westronics, Inc. of Ft. Worth, Tex. and bears model No.YSDl l-E.

Trip time (t,) 6, bit cost (C 14 and rig cost (C,.) 10 are entered intothe computer apparatus as constants for any one computation. Theinternal variable, bit rotating time (t,), is

supplied by rotating time circuit 2. The output of circuit 2 is a seriesof pulses or clock signals representing a specific time interval. Forexample, one clock pulse might represent onetenth of an hour. The clocksignals are directed via conductor 3 to storage circuit 4 foraccumulation and conversion to a proportionate analog DC voltage. A moredetailed diagram of circuit 4 is shown in FIG. 3. Time pulses viaconductor 3 are accumulated in digital register 27. The binary output ofdigital register 27 is applied through conductor 28 to digital-toanalogconverter 29. The output of D-to-A converter 29 is applied via conductor30 to buffer amplifier 31. Amplifier 31 supplies a DC voltage directlyproportional to the number of time pulses accumulated by digitalregister 27. Rotating time voltage is then applied via conductor 5 toadder 8.

Trip time (n) is set into the computer by a direct reading potentiometercontrol in circuit 6. A typical circuit of this type is shown in FIG. 4.The output of circuit 6 is an analog voltage which is directlyproportional to trip time. Trip time voltage is applied via conductor 7to a second input of adder 8. The output of adder 8 is equal to:

T= t, +1, where T total of rotating time plus trip time. The total time(T) is directed via conductor 9 to multiplier 12. The second input tomultiplier 12 is supplied on conductor 11 from rig cost circuit 10. Thiscircuit may be the same as trip time circuit 6 shown in FIG. 4, with anappropriate scale on potentiometer R being provided to allow a directsetting of rig cost. The output of multiplier 12 is an analog DC voltageproportional to:

r (r i) This output of multiplier 12 is then applied via conductor 13 toadder 16.

Bit cost (C,,) is manually dialed into the computer by means of a directreading potentiometer control in circuit 14. This circuit is the same astrip time circuit 6 with an appropriate scale for potentiometer controlR: so that the analog DC output voltage is directly proportional to thedesired bit cost in dollars. The output of circuit 14 is directed viaconductor 15 to the second input of adder 16. Adder 16 combines thesignals from multiplier 12 and bit cost circuit 14 to provide an outputsignal proportional to the total rig cost plus bit cost, or:

rflrds v This voltage representing total cost is directed via conductor17 to one input of analog divider 21.

Depth pulses 18 from the depth retransmission device (not shown) aredirected to storage circuit 19 and to strip chart recorder 33. Storagecircuit 19 may be identical to storage cir cuit 4 and is shown in moredetail in FIG. 3. In storage circuit 19, depth pulses are accumulated indigital register 27. The binary output of register 27 is directed viaconductor 28 to digital-to-analog converter 29. The output of D-to-Aconverter 29 via conductor 30 and buffer amplifier 31 represents ananalog voltage whose absolute value is directly proportional toaccumulated depth. The analog output of storage unit 19 is directed viaconductor 20 to the second input of analog divider 21. The output ofdivider 21 represents the desired average cost per foot which is:

The analog voltage representing cost per foot is directed via conductor22 to scaling amplifier circuits 23. One output of circuit 23 is routedvia conductor to analog meter 26, providing an instantaneous readout orindication of average cost. The second output of circuit 23 is routedvia conductor 24 to the vertical or Y-input of strip chart recorder 33.The horizontal or X-input of strip chart recorder 33 is pulse activatedor driven by depth pulses 18 such that the chart is advanced for eachpulse or unit of depth. Use of the described incremental advance stripchart recorder allows the curve of FIG. 1 to be automatically plotted.In addition, a wide range of resolution may be obtained by properscaling of computer inputs and recorder functions.

In operation, the time required to trip the drill string out of theborehole, replace the drill bit and return the bit to the boreholebottom is measured and entered by the drilling operator in circuit 6. Atthe same time, the cost of the new bit is entered into circuit 14 andthe hourly drilling apparatus cost is entered into circuit 10. Asdrilling begins, rotating time circuit 2 is activated to produce clockpulses which are stored in circuit 4 and a depth recorder (not shown)produces depth pulses 18 which are stored in circuit 19. As describedabove, the computer apparatus thereafter continuously calculates averagedrilling cost in dollars per foot, which figure is applicable to theinterval drilled with the new drill bit. This average drilling costfigure is instantaneously indicated to the drilling operator on meter 26and is recorded as a function of depth drilled during the interval onrecorder 33. Because the drill bit is consumable and loses drillingefficiency after a period of use, the curve produced by recorder 33 willapproximate that shown in FIG. 1, with average drilling cost decreasingfrom point A to point B and increasing thereafter. By examining thechart or by monitoring meter 26, the drilling operator can detenninewhen the point of maximum drilling efficiency is passed and a new drillbit is required.

The method of this invention may be practiced using the apparatusdescribed above. A depth recorder (not shown) senses incrementalincreases in depth during the interval of drilling with each bit andproduces signals 18 representative thereof. Circuits 2 through 16produce a second signal on conductor 17 which is representative of thetotal operating cost during the time necessary to drill the interval ofdepth. The signal on conductor 17 is then divided by the depth signal 18that has been stored in circuit 19 to produce an output signal onconductor 22 representative of the average cost of operating thedrilling apparatus for each incremental depth of borehole drilled duringthe drilling interval. This output signal may be monitored with meter 26and may be recorded on strip chart recorder 33.

The foregoing is to be construed as illustrative only and furthermodifications and alternate equivalent embodiments will be obvious tothose skilled in the art in view of this description.

What is claimed is:

1. Apparatus for determining drilling cost while drilling a borehole,comprising:

first means for sensing successive incremental lengths of an interval ofborehole depth drilled and generating first electrical signalsrepresentative thereof;

second electrical means for generating a second signal representative ofaccumulated drilling cost incurred as said interval of depth is drilled;

and, third means responsive to said first and second signals forgenerating a third electrical signal representative of average operatingcost per incremental length of borehole drilled over said interval ofdepth.

2. The invention as claimed in claim 1 including:

means responsive to said third signal for reading out the average costper incremental length of borehole drilled.

3. The invention as claimed in claim 1 wherein said second meansincludes:

fourth electrical means for generating a fourth signal representative ofthe length of time said drilling apparatus is utilized in boring saidinterval of depth;

and, fifth means for generating a fifth electrical signal representativeof the cost of operating said drilling apparatus during said length oftime.

4. The invention as claimed in claim 3 wherein said fourth meansincludes:

sixth electrical means for generating a sixth signal representative ofthe rotating time of said drilling apparatus;

and, seventh electrical means for generating a seventh signalrepresentative of the amount of time in excess of rotating time thatsaid drilling apparatus is utilized in boring said interval of depth.

5. The invention as claimed in claim 4 including:

eighth electrical means for generating an eighth signal representativeof the cost of the drill bit utilized in drilling said interval ofdepth.

6. The invention as claimed in claim 5 including:

means responsive to said third signal for reading out the average costper incremental length of borehole drilled.

7. The invention as claimed in claim 5 including:

means for graphically recording said average drilling cost perincremental length of said interval of depth as a function of the depthdrilled during said interval.

8. The method of determining drilling costsincluding the steps of:

sensing successive incremental lengths of an interval of borehole depthdrilled and generating a first electrical signal representative thereof;

generating a second electrical signal representative of accumulateddrilling cost incurred as said interval of depth is drilled;

and, generating a third electrical signal responsive to said first andsecond signal representative of average operating cost per incrementallength of borehole drilled over said interval of depth.

9. The invention as claimed in claim 8 including:

reading out said third signalas an indication of the average cost perincremental length of borehole drilled.

10. The invention as claimed in claim 8 wherein said step of generatingsaid second signal includes:

generating a fourth electrical signal representative of the length oftime said drilling apparatus is utilized in boring said interval ofdepth;

and, generating a fifth electrical signal representative of the cost ofoperating said drilling apparatus during said length of time.

11. The invention as claimed in claim 10 wherein said step of generatingsaid fourth signal includes:

generating a sixth electrical signal representative of the length oftime said drilling apparatus is rotating a drill bit in said borehole;

and, generating a seventh electrical signal representative of the lengthof time in excess of said rotating time that said drilling apparatus isutilized in boring said interval of depth.

12. The invention as claimed in claim 11 including:

generating an eighth electrical signal representative of the cost of thedrill bit utilized in drilling said borehole during said interval ofdepth.

13. The invention as claimed in claim 12 including:

indicating in response to said third signal the average cost perincremental length of borehole drilled.

14. The invention as claimed in claim 12 including:

graphically recording said average drilling cost per incremental lengthof said interval of depth as a function of the depth drilled during saidinterval.

l 4' II I I U. S. PATENT OFFICE v 1 UNITED STATES PATENT OFFICECEII'IIFICATE OF CORRECTION Patent No. Dated May 972 It is certifiedthat error appears in the above-identified patent and that said LettersPatent are hereby corrected as shown below:

' Column. 2, line 10;

Column 2, line IO;

Column 3, line 60;

should be Signed and sealed this 29th .cay of August 1972.

(SEAL) Attest:

ROBERT GOTTSCHALK Commissioner of Patents EDWARD M.FLETCHER,JR.Attesting Officer

1. Apparatus for determining drilling cost while drilling a borehole,comprising: first means for sensing successive incremental lengths of aninterval of borehole depth drilled and generating first electricalsignals representative thereof; second electrical means for generating asecond signal representative of accumulated drilling cost incurred assaid interval of depth is drilled; and, third means responsive to saidfirst and second signals for generating a third electrical signalrepresentative of average operating cost per incremental length ofborehole drilled over said interval of depth.
 2. The invention asclaimed in claim 1 including: means responsive to said third signal forreading out the average cost per incremental length of borehole drilled.3. The invention as claimed in claim 1 wherein said second meansincludes: fourth electrical means for generating a fourth signalrepresentative of the length of time said drilling apparatus is utilizedin boring said interval of depth; and, fifth means for generating afifth electrical signal representative of the cost of operating saiddrilling apparatus during said length of time.
 4. The invention asclaimed in claim 3 wherein said fourth means includes: sixth electricalmeans for generating a sixth signal representative of the rotating timeof said drilling apparatus; and, seventh electrical means for generatinga seventh signal representative of the amount of time in excess ofrotating time that said drilling apparatus is utilized in boring saidinterval of depth.
 5. The invention as claimed in claim 4 including:eighth electrical means for generating an eighth signal representativeof the cost of the drill bit utilized in drilling said interval ofdepth.
 6. The invention as claimed in claim 5 including: meansresponsive to said third signal for reading out the average cost perincremental length of borehole drilled.
 7. The invention as claimed inclaim 5 including: means for graphically recording said average drillingcost per incremental length of said interval of depth as a function ofthe depth drilled during said interval.
 8. The method of determiningdrilling costs including the steps of: sensing successive incrementallengths of an interval of borehole depth drilled and generating a firstelectrical signal representative thereof; generating a second electricalsignal representative of accumulated drilling cost incurred as saidinterval of depth is drilled; and, generating a third electrical signalresponsive to said first and second signal representative of averageoperating cost per incremental length of borehole drilled over saidinterval of depth.
 9. The invention as claimed in claim 8 including:reading out said third signal as an indication of the average cost perincremental length of borehole drilled.
 10. The invention as claimed inclaim 8 wherein said step of generating said second signal includes:generating a fourth electrical signal representative of the length oftime said drilling apparatus is utilized in boring said interval ofdepth; and, generating a fifth electrical signal representative of thecost of operating said drilling apparatus during said length of time.11. The invention as claimed in claim 10 wherein said step of generatingsaid fourth signal includes: generating a sixth electrical signalrepresentative of the length of time said drilling apparatus is rotatinga drill bit in said borehole; and, generating a seventh electricalsignal representative of the length of time in excess of said rotatingtime that said drilling apparatus is utilized in boring said interval ofdepth.
 12. The invention as claimed in claim 11 including: generating aneighth electrical signal representative of the cost of the drill bitutilized in drilling said borehole during said interval of depth. 13.The invention as claimed in claim 12 including: indicating in responseto said third signal the average cost per incremental length of boreholedrilled.
 14. The invention as claimed in claim 12 including: graphicallyrecording said average drilling cost per incremental length of saidinterval of depth as a function of the depth drilled during saidinterval.